Row Level Security by Group in Table

Hi Everyone,

if got a slight problem using RLS in a report.

I identify my users via userprincipalname() and their email adress, which works pretty fine if I want to show the user only the data he's created.

But now I've got to build a second level of security based on another row in the report to give a group view of the same data.

For Example:

The requirement is now, that every User should see all data for his group, not only the data which is assigned to the user specific, so User A should also be allowed to see Data of User B.

These User-Groups are not defined in the AD, only in my data tables (which come from HR)

Is there a simple way of doing so? 

I'm pretty new to RLS...